Anonymity. Now the VPN operators and people who are watching the VPN logs don't know your IP.
all VPNs need authentication and payments, otherwise your just connecting to another open network with the same issues as TOR.
if you make payments, and then auth on connection, they "know" who you are and TOR at the start is not giving you anything
2. Not all payment methods are connected to your identity. Mullvad for example accepts cash in an envelope, or cryptocurrency payments.